1. Field of the Invention
The present invention relates to chemical vapor deposition apparatuses, and more particularly relates to a chemical vapor deposition apparatus for producing a dielectric thin film such as a semiconductor memory.
2. Description of the Background Art
Higher integration of a semiconductor memory or device is progressing rapidly. The number of bits of the dynamic random access memory (DRAM) exhibits a fourfold increase in three years. This is the result of pursuing a goal of higher integration, lower power consumption, and lower cost of the device. Even if the degree of integration is improved, a capacitor as one component of the DRAM should have a certain capacitance. Although the film thickness of the material for the capacitor insulating film should be decreased, the SiO.sub.2 which is heretofor used has limitations in decreasing its film thickness.
If the dielectric constant of the capacitor insulating film can be increased by utilizing another material, both of a thinner thickness of the film and enough capacitance can be attained. Accordingly, respective companies are carrying out an investigation on the use of a thin film formed of high dielectric material for the capacitor insulating film of the memory device.
The most important performances required of such a capacitor insulating film are that it is a thin film of a high dielectric constant and that the leakage current is small. In other words, as long as the high dielectric constant material is utilized, it should be as thin as possible and the leakage current should be as small as possible. As goals in the development, a film thickness of at most 0.5 nm as converted to SiO.sub.2 equivalent, and leakage current density of at most 2.times.10.sup.-7 A/cm.sup.2 when a voltage of 1V is applied are generally desirable.
In order to form a thin film over an electrode for the capacitor of the DRAM having steps, the chemical vapor deposition (CVD) is advantageous since the coverage of a surface having a complex shape is excellent. According to the CVD, a liquid source obtained by dissolving an organometallic complex containing a specific metal in an organic solvent is used as the material for a thin film having a high dielectric constant. By vaporizing the liquid source and dispersing the obtained gas over a substrate or the like, a thin film of a high dielectric constant is formed.
A serious problem in this case is that there is no liquid source having a stable and excellent vaporization property. The reason is that a compound of dipivaloylmethane (DPM) of .beta.-diketone system with metal which is mainly employed as the organometallic complex does not exhibit a good vaporization property when heated.
The inventors of this application suggested to use a CVD material which is a liquid source obtained by dissolving the organometallic complex in an organic solvent of tetrahydrofuran (THF), and thus acquired a significantly improved vaporization property in Japanese Patent Laying-Open No. 6-158328. However, even if the liquid source above is applied to a conventional CVD apparatus used for the liquid source to produce a dielectric film, desirable results are not necessarily produced.
The inventors accordingly suggested a CVD apparatus for the liquid source in which the liquid source is vaporized enough to be stably supplied to a reaction chamber (Japanese Patent Laying-Open Nos. 6-310444 and 7-094426).
Generally, TTIP[Ti(O--i--Pr).sub.4 ] is utilized as the organometallic complex containing titanium as the metal. According to Japanese Patent Laying-open No. 7-268634, TiO (DPM).sub.2 of DPM system is used so that a highly improved coverage of a thin film can be obtained. Further, a process of forming a film consisting of two steps of crystallizing an initial film by annealing, and thereafter depositing a film of the second layer is effective in order to obtain surface morphology and an electrical property superior to a single layer film.
A CVD apparatus for the liquid source is further proposed which is provided with the FTIR or the like for optically monitoring the process of forming a film in-situ. A lower electrode structure is further proposed which is suitable for the BST [(Ba, Sr) TiO.sub.3 ] film produced by a CVD process in which the liquid source is vaporized (Japanese Patent Laying-Open Nos. 8-176826 and 8-186103).
However, the apparatus for liquid source CVD does not provide formation of a film having a good property (including an electrical property) when it is used as an apparatus for producing a dielectric film.
A conventional apparatus for liquid source CVD is hereinafter described. Referring to FIG. 15, the apparatus for liquid source CVD includes a CVD source container 103, a vaporizer 105, and a reaction chamber 111. CVD source container 103 stores a liquid CVD source obtained by dissolving an organometallic complex containing a prescribed metal in an organic solvent. Vaporizer 105 is used for vaporizing the liquid CVD source delivered from a source supply unit 115. The vaporized CVD source is mixed with oxygen as an oxidizer in a mixer portion 107. The oxygen is transported through an oxidizer supply pipe 121. The CVD source produced by the vaporization and mixing with oxygen is dispersed over a substrate 111c fron a gas nozzle 111d. Substrate 111c is mounted on a stage 111b in a reaction chamber 111e.
A process of forming a film utilizing the above apparatus for liquid source CVD is described below. A pressure gas such as nitrogen is introduced from a pressure pipe 113 into CVD source container 103. The pressure in CVD source container 103 accordingly increases and the liquid CVD source is delivered to vaporizer 105. At this time, the flow rate of the CVD source is controlled by source supply unit 115. Further, nitrogen is introduced from a carrier gas introduction pipe 104. The liquid CVD source is sprayed into a vaporize chamber within the vaporizer and vaporized to become a CVD source gas.
The CVD source gas passes through a source gas transport pipe 117 and arrives at mixer portion 107. A transport pipe heater 141 provided around source gas transport pipe 117 prevents the CVD source gas from liquefying. A vent line 152 is connected to source gas transport pipe 117. In mixer portion 107, the CVD source gas is mixed with oxygen. The CVD source gas mixed with oxygen is spouted from gas nozzle 111d. A thin film is thus formed on the substrate heated by a substrate heater.
Organometallic complexes respectively containing Ba, Sr, and Ti are dissolved in the organic solvent and used as the liquid source. Although only one line of CVD source container 103 is shown in FIG. 15, actually three lines of CVD source containers 103 are provided. The liquid CVD source is supplied from respective CVD source containers to one vaporizer 105.
Reaction chamber 111 has an oxygen ambience and a pressure of 1-10 Torr. The temperature of the substrate heater is set at 400-600.degree. C. since an excellent coverage can be obtained when the temperature at formation of a film is relatively low. The flow rate of the liquid source and the time for the formation of a film are controlled such that the rate of the film formation is 30 .ANG./min and the film thickness is 300 .ANG.. A film formed on the substrate is the BST film [(Ba, Sr) TiO.sub.3 ]. The film is produced such that the ratio of composition of the BST film, (Ba+Sr)/Ti, is 1.0.
A sample is prepared by forming the BST film on a lower electrode such as Pt or Ru, and forming an upper electrode such as Pt or Ru on the BST film. Utilizing the sample, an electrical property of the BST film such as the leakage current or the film thickness as converted to an oxide film equivalent is measured.
In order to produce the BST film by the apparatus for liquid source CVD described above, a liquid source obtained by dissolving an organometallic compound of DPM system in an organic solvent is used as a CVD material. A gas-liquid mixture of the liquid source and the carrier gas is sprayed from a Teflon nozzle having an outer diameter of 1/16 inches to a vaporization system having a temperature of 250.degree. C. At this time, condensation of the liquid source causes clogging of the tip portion of the nozzle. Repetition of the film formation generates some residues on an inner side face of the vaporize chamber. As a result, BST films having the same composition and film thickness cannot be produced stably.
The residue may stick to an inner wall of a pipe connecting a vaporizer and a reaction chamber. If the residue enters the reaction chamber and is taken into a film during the film formation process, the film quality deteriorates.
A source gas is mixed with oxygen as an oxidizer in the mixer portion. If the temperature of the oxygen does not increase enough, the source gas is cooled down to be condensed in the mixer portion. As a result, clogging of the mixer portion could occur.
Each time a film is formed, the residue is generated inside the mixer portion. The oxygen having a relatively lower temperature diffuses back into a source gas transport pipe so that the source gas is cooled to be condensed during formation of a film. As a result, stable formation of films having the same composition and thickness cannot be provided.
When the source gas is introduced into a reaction chamber through a gas nozzle, the temperature of the gas nozzle should be kept at a temperature which is at least the sublimation temperature or the boiling point of the CVD source. Especially a CVD source of the organometallic compound of DPM system such as Ba(DPM).sub.2 is suffered from pylolysis if its temperature rises to a point approximately 10-20.degree. C. higher than the sublimation temperature or the boiling point. Therefore, the temperature of the gas introduction system should be controlled precisely.
However, especially the temperature of the edge portion of the gas nozzle of a gas head often rises above a prescribed temperature due to the heat radiated from a substrate heated to approximately 400-600.degree. C. On the other hand, the temperature of the edge portion of the gas nozzle sometimes falls below a prescribed temperature due to the radiation or conduction of the heat from the gas nozzle to a chamber having a relatively low temperature. As a result, the vaporized CVD source is decomposited or solidified. Further, holes of the gas nozzle could be clogged.
During formation of the film, if a reaction product sticking to an inner wall of the reaction chamber is peeled off onto the substrate, the film quality would deteriorate.
In the conventional apparatus for liquid source CVD, contaminants such as residues of the CVD source stick to the vaporize chamber, the nozzle, the source transport pipe or the like. Contaminants such as the reaction product or the like stick to the inner wall of the reaction chamber. Consequently, a CVD source gas having a prescribed flow rate or component often fails to be dispersed over the substrate. Further, some contaminants drop onto the substrate. As a result, stable formation of a desired thin film on a substrate becomes impossible.